An uninterruptible power supply (UPS) may be used to provide emergency power in situations where the main power supply fails or performs in an unusual manner. A UPS is designed to switch over as quickly as possible, given limitations of the batteries used and the circuitry of the UPS. A UPS can be used to deal with a number of unusual events occurring at the main power supply, such as: power failure; surge; sag; spikes; noise; frequency instability; harmonic distortion; etc.
A UPS can be used to protect any type of equipment, however, generally a UPS is most often found in computers, data centers, telecommunication systems, and any other electrical equipment for which a power supply failure could cause serious consequences such as damage to a person or to a business interest.
A UPS can take many different forms and relate to various different technologies. The most common general categories of UPS are online, line interactive, and standby. Each of these is well-known in the art as are the other alternatives such as hybrid topologies and ferro-resonant technologies.
An essential element of a UPS is a battery which provides the power after switch over has occurred. The capacity of the battery determines the period of “autonomy” which the UPS can provide for a load, in the absence of a mains supply. Batteries may be required for example to provide a nominal power output over a period of, say, 8 hours. Provided the external (mains) supply is restored within that period, power to the load is uninterrupted. However, once the power is restored, the battery will be in a (partially) discharged state. Consequently, until the battery is recharged to a full state, the autonomy offered by the UPS is reduced below the 8 hour period. A second incident interrupting the main supply brings much greater risk to the collected loads.
Two factors that limit the speed of recovery of charge, and hence the recovery of autonomy, are the battery technology itself and the rate at which charging current can be supplied by the UPS to recharge the battery. The term “battery” as used herein should be understood to encompass any arrangement in which energy can be stored while being received and released in the form of electricity. Typically, lead acid batteries are used. Traditional lead acid batteries are limited to a slow charging rate, such that a fully discharged battery may take, for example, 10 hours to recharge. More modern battery technologies, for example lithium iron phosphate (LiFePO4) batteries, can be recharged much more quickly, offering the potential to reduce recovery times in UPS systems. However, to recharge the battery so quickly requires a proportionately higher charging current. Using current UPS architectures, to provide such a high charging current whilst simultaneously providing a conditioned supply to the load under normal, mains-powered operation, requires significantly increased power ratings in the converter circuits that are the key components of a UPS circuit. To increase the rating of the circuits, for example to provide recharging of the battery at a rate equal to the maximum rate at which it can be discharged, an input converter of the UPS would require a doubled power rating. This has a disproportionate impact on cost of the circuitry, and discourages exploitation of the potential of these new battery types.